Deciphering size-induced influence of carbon dots on mechanical performance of cement composites

Carbon dots (CDs) as a newfangled and eco-friendly nanomaterial exhibit a promising engineering application in strengthening mechanical properties of cement matrices. However, the modification effectiveness of CDs on mechanical performance lacks enough investigation. More seriously, the influence of their sizes is still a research gap, enormously hindering their further application in cement-based materials. Herein, the size-induced effect of CDs on mechanical properties of cement mortar is systematically disclosed for the first time. Specifically, three kinds of CDs with diverse sizes are deliberately synthesized via calcinating the cheap and nontoxic raw material (i.e., citric acid). The relationship between the mechanical performance and the sizes of CDs is determined through the compressive strength test. More importantly, the mechanism of their size-induced effect on mechanical properties is comprehensively studied based on the analyses of phase composition and pore structure. The consequences illustrate that the compressive strength of mortar modified by CDs with average sizes of 12.53, 5.86, and 1.81 nm increases by 6.6%, 15.7% and 9.4% separately, compared to the blank group after 7-d curing. With the sizes of CDs reducing ranging from 12.5 to 1.81 nm, the compressive strength of CDs-modified mortar firstly increases and then decreases. The related mechanism is attributed to the combined effect of CDs-induced nucleation and air entrainment. As the sizes of CDs decrease, the intensities of both the nucleation effect and air entrainment effect gradually strengthen, thereby accelerating the cement hydration to generate more C-S-H and introducing the harmful pores. This work fills the research gap of the effect caused by CDs sizes on mechanical properties for cement-based materials. This would guide preparation of CDs with suitable sizes toward increasing mechanical properties of cement matrices, thus facilitating the development of high-performance concrete by CDs-based nanomodification.